Method: 1. Cut the fish fillets into small/medium squares. 2. Mix the nigella seeds, red chilli powder, turmeric powder, 2 tsp salt and 3 to 4 tblsp mustard oil in a bowl. 3. Add the fish pieces and mix until evenly coated. 4. Cover and keep them in the fridge for 2 hours. 5. Remove the fish from the fridge and mix with sugar, mustard paste and remaining oil. 6. Stir gently, cover and keep it back in the fridge for 1 hour. 7. Gently warm the banana leaves over an open flame for 30 seconds, ensuring it does not get burnt. 8. Add the marinated fish pieces to the banana leaves and fold them well. 9. Secure the edges with toothpicks and steam cook for 20 to 30 minutes. 10. Serve hot.